There is a common perception that smoking generally helps peopleto manage stress, and may be a form of 'self-medication' in people with mental health conditions.However, there are biologically plausible reasons why smoking may worsen mental health through neuroadaptations arising from chronic smoking, leading to frequentnicotinewithdrawalsymptoms(e.g. anxiety, depression, irritability),in which case smoking cessation may help to improve rather than worsen mental health.
To examine the association between tobacco smoking cessation and change in mental health.
We searched the Cochrane Tobacco Addiction Group's Specialised Register, Cochrane Central Register of Controlled Trials, MEDLINE, Embase, PsycINFO, and thetrial registries clinicaltrials.gov and the International Clinical Trials Registry Platform,from 14April 2012to 07 January 2020.These wereupdated searchesof apreviously-conducted non-Cochrane review where searches wereconductedfrom database inceptionto 13 April 2012. SELECTION CRITERIA: We included controlled before-after studies, including randomised controlled trials (RCTs) analysed by smoking status at follow-up, and longitudinal cohort studies. In order to be eligible for inclusion studies had to recruit adults who smokedtobacco, and assess whether they quit or continued smoking during the study. They also had to measure a mental health outcome at baseline and at least six weeks later.
We followed standard Cochrane methods for screening and data extraction. Our primary outcomes werechange in depression symptoms, anxiety symptoms or mixed anxiety and depression symptoms between baseline and follow-up. Secondary outcomes includedchange in symptoms of stress, psychological quality of life, positive affect, and social impact or social quality of life, as well as new incidence of depression, anxiety, or mixed anxiety and depression disorders. We assessed the risk of bias for the primary outcomesusing a modified ROBINS-I tool.For change in mental health outcomes, we calculated the pooled standardised mean difference (SMD) and 95% confidence interval (95% CI) for the difference in change in mental health from baseline to follow-up between those who had quit smoking and those who had continued to smoke. Forthe incidence of psychological disorders, we calculated odds ratios (ORs) and 95% CIs. For all meta-analyses we used a generic inverse variance random-effects model and quantified statistical heterogeneity using I2. We conducted subgroup analyses to investigate any differences in associations between sub-populations, i.e. unselected people with mental illness, people with physical chronic diseases. We assessed the certainty of evidence for our primary outcomes (depression, anxiety, and mixed depression and anxiety) and our secondary social impact outcomeusingthe eight GRADE considerations relevant to non-randomised studies (risk of bias, inconsistency, imprecision, indirectness, publication bias, magnitude of the effect, the influence of all plausible residual confounding, the presence of a dose-response gradient).
We included 102 studies representing over 169,500 participants. Sixty-two of these were identified in the updated search for this review and 40 were included in theoriginal version of thereview. Sixty-three studies provided data onchange in mental health, 10 were included in meta-analyses of incidence of mental health disorders, and 31 were synthesised narratively. For all primary outcomes, smoking cessation was associated with an improvement in mental health symptoms compared with continuing to smoke: anxiety symptoms(SMD -0.28, 95% CI -0.43 to -0.13; 15 studies, 3141 participants; I2 = 69%; low-certainty evidence);depressionsymptoms: (SMD -0.30, 95% CI -0.39 to -0.21;34 studies, 7156 participants; I2 = 69%' very low-certainty evidence);mixed anxiety and depression symptoms(SMD -0.31, 95% CI -0.40 to -0.22; 8 studies, 2829 participants; I2 = 0%; moderate certainty evidence). These findings were robust to preplanned sensitivity analyses, and subgroup analysis generally did not produce evidence of differences in the effect sizeamong subpopulations or based on methodological characteristics.All studies were deemed to be at serious risk of bias due to possible time-varying confounding, and three studiesmeasuring depression symptoms werejudged to be at criticalrisk of bias overall. There was also some evidence of funnel plot asymmetry. For these reasons, we rated our certainty in the estimates for anxiety as low, for depression as very low, and for mixed anxiety and depression as moderate. For the secondary outcomes, smoking cessation was associated with an improvement in symptoms of stress(SMD -0.19, 95% CI -0.34 to -0.04; 4 studies, 1792 participants; I2 = 50%), positive affect (SMD 0.22, 95% CI 0.11 to 0.33; 13 studies, 4880 participants; I2 = 75%), and psychological quality of life (SMD 0.11, 95% CI 0.06 to 0.16; 19 studies, 18,034 participants; I2 = 42%). There was also evidence that smoking cessation was not associated with areduction in social quality of life, with the confidence interval incorporating the possibility of a small improvement(SMD 0.03, 95% CI 0.00 to 0.06; 9 studies, 14,673 participants; I2 = 0%). The incidence of new mixed anxiety and depression was lower in people who stopped smoking compared with those who continued (OR 0.76, 95% CI 0.66 to 0.86; 3 studies, 8685 participants; I2 = 57%), as was the incidence of anxiety disorder (OR 0.61, 95% CI 0.34 to 1.12; 2 studies, 2293 participants; I2 = 46%). We deemed it inappropriate to present a pooled estimate for the incidence of new cases of clinical depression, as there was high statistical heterogeneity (I2 = 87%).
Taken together, these data provide evidence that mental health does not worsen as a result of quitting smoking, and very low- to moderate-certainty evidence that smoking cessation is associatedwithsmall to moderate improvements in mental health. These improvements are seen in both unselected samples and in subpopulations, including people diagnosed with mental health conditions.Additional studies that use more advanced methods to overcome time-varying confounding would strengthen the evidence in this area.